1. Field of the Invention
The present invention relates to a receiving technology, and it particularly relates to a receiving method for receiving signals by a plurality of antennas and a receiving apparatus and a radio apparatus using said method.
2. Description of the Related Art
In wireless communications, it is generally desired that the limited frequency resources be used effectively. One of the technologies that effectively utilize the frequency resources is adaptive array antenna technology. In the adaptive array antenna technology, the amplitude and phase of signals to be received and transmitted by a plurality of antennas are so controlled as to form directional patterns of the antennas. That is, an apparatus provided with adaptive array antennas varies respectively the amplitude and phase of signals received by a plurality of antennas and sums up a plurality of the thus varied received signals. This corresponds to receiving the signals equivalent to the signals received by the antenna having the directional pattern corresponding to a variation in said amplitude and phase (hereinafter referred to as “weight”). Also, the signals are transmitted in the directional pattern of the antenna corresponding to the weight.
One example of weight computation in the adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be a condition for an optimum weight value. Also known is a recurrence formula whose amount of calculation is smaller than directly obtaining the Wiener solution. For such a recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used.
In general, there is phase error called “frequency offset” between a signal oscillated by a local oscillator included in a transmitting apparatus and a signal oscillated by a local oscillator included in a receiving apparatus. For example, in a case when QPSK (Quadrature Phase Shift Keying) or the like is used as a modulation scheme preset between the transmitting apparatus and the receiving apparatus, the constellation of signals received by the receiving apparatus is rotated due to the phase error. Since such a rotation of signal points degrades the transmission quality of signals, the receiving apparatus is usually provided with an AFC (Automatic Frequency Controller) to prevent this undesired rotation.
In adaptive algorithms and the like, weights are generally computed in a known reference signal period and are combined by weighting data signals subsequent to the reference signal. However, a plurality of local oscillators are provided respectively for a plurality of antennas constituting an adaptive array. If the frequency stability of a plurality of local oscillators is low, a difference of phase error between a plurality of signal periods will be larger with time. As a result, it may well be that a plurality of signals, which can be in-phase combined in a reference signal period, will not be in-phase combined in an end of data signal. When an OFDM (Orthogonal Frequency Division Multiplex) modulation scheme is used, a combining gain drops due to such phase error and therefore the transmission quality of signals deteriorates significantly. Also, the phase error differs on a subcarrier-by-subcarrier basis due to Doppler shift or frequency selective fading. In order to reduce such phase error, the weight may be adaptively updated even after the reference signal period has elapsed. However, a method for adaptively updating the weight generally entails a rise in calculation amount, thus leading to an increase in circuit scale and a rise in circuit cost. Hereinbelow, differences in antenna-related frequency offset over a plurality of antennas are called “interantenna frequency offset error”.